1. Field of the Invention
The present invention relates to a layer forming a barrier to ionic bombardment for a vacuum tube.
2. Description of the Prior Art
It is known to use such layers, more especially in light image intensifier tubes.
Thus the English patent No. 1,368,882 relates to a light image intensifier tube having a slab of microchannels having on the photocathode side of the tube a layer forming a barrier to ionic bombardment. Such a tube is shown schematically in FIG. 1.
In this FIGURE, the reference 1 designates the glass plate which receives the light radiation shown symbolically by a wavy arrow.
Following this glass plate we find a photocathode 2 which is coated with a layer 3 allowing extraction of photoelectrons. We then find the ionic bombardment barrier layer 4 which is carried by the slab of microchannels 5, then a metal layer 6, a luminescent material layer 7 and a glass plate 8.
The residual pressure in a vacuum tube is never zero whatever the quality of cleaning and degassing of its component parts. The electronic bombardment ionizes the residual gases and the ions thus created, charged positively travel up the channels of the slab to the lowest potential and bombard the extraction layer 3 situated at the surface of the photocathode. This extraction layer, which is very fragile and very thin, is very rapidly destroyed by the ionic bombardment.
To avoid such destruction, an ionic bombardment barrier layer 4 is used, laid on the slab of microchannels 5, on the photocathode side. This layer stops the ions but lets through the photoelectrons which are suitably accelerated by a sufficient potential.
In the cited English patent, the process used for depositing the barrier layer is the following:
a thin organic film, with a nitrocellulose base for example, is formed using one of the processes for manufacturing cathode ray tube screens; PA1 this film is deposited on the microchannel slab; PA1 a protective material layer, preferably a layer of aluminium, is deposited on the organic film laid on the microchannel slab; PA1 air annealing is carried out so as to ensure combustion of the organic film and to ensure a contact between the protective material layer and the microchannel slab. PA1 the organic film may be damaged; PA1 rotating pieces and sophisticated regulations are required for forming very thin films in a reproducible way; PA1 finally, heating during the different depositions is difficult to provide. PA1 good adhesion to the microchannel slab; PA1 the absence of degassing under ionic bombardment; PA1 a mean atomic mass small enough for the photoelectrons to be able to pass readily therethrough; PA1 the absence of defects such as holes or tears resulting either from the annealing and the stress state which is created or from the handlings required, for example, for fitting the microchannel slab in the light image booster.
When an aluminium barrier layer is used, adhesion of the barrier layer to the microchannel slab and the stress state of the barrier layer after annealing are not very satisfactory.
On the other hand, aluminium is a material easy to deposit and which does not give rise to degassing when it is bombarded.
For forming this barrier layer, it is known to use other materials such as alumina Al.sub.2 O.sub.3, a silicon oxide SiO.sub.2 or SiO, or zinc sulphide ZnS. None of these materials gives full satisfaction. Alumina has a stress state after annealing which is not very satisfactory, silicon oxide SiO.sub.2 or SiO degasses when it is bombarded, and zinc sulphide has a mean atomic mass which is too larger for the photoelectrons to be able to pass readily therethrough.
In the prior art, the ionic bombardment barrier layers are deposited by well known techniques of cathode spraying or Joule effect evaporation. Such techniques have more particularly the following disadvantages: